The present invention relates generally to threaded fasteners and, more particularly, to internally threaded prevailing-torque type self-locking fasteners and a method for their manufacture.
Prevailing-torque self-locking fasteners are well known in the art. These fasteners are frictionally resistant to rotation due to a self-contained prevailing-torque feature, often called a "locking element." A variety of different prevailing-torque fasteners are known, including so-called "all metal" fasteners and "plastic patch," "plastic strip" or "plastic pellet" fasteners. As their respective names imply, these fasteners achieve a self-locking or prevailing-torque function due to the characteristics of the particular locking element employed.
The present invention is directed only to prevailing-torque type, self-locking fasteners that employ a patch of thermoplastic material as the locking element. Moreover, the invention is directed to internally threaded fasteners. Such patch type internally threaded self-locking fasteners are well known in the art and a variety of machines and processes for their manufacture are also known, as disclosed, for example in U.S. Pat. Nos. 3,858,262; 3,894,509; 3,995,074; 4,054,688; 4,060,868; 4,100,882; 4,366,190; 4,775,555; 5,141,771; 5,221,170; 5,511,510; 5,620,520; and 5,718,945. The disclosures of these listed U.S. Patents are incorporated herein by reference.
It is known that a large market exists for internally threaded self-locking fasteners having relatively stringent functional specifications. That is, the torque required to achieve a particular clamp load between mated internally and externally threaded fasteners must be no more than a specified maximum, while the torque required to disengage the mated fasteners must exceed a specified minimum. The Industrial Fasteners Institute ("IFI") has promulgated standards relevant to such specifications. For example, IFI 100/107 (1987) defines a maximum installation torque that cannot be exceeded and minimum removal torques (both for the first and fifth removals) that must be exceeded. Similarly, IFI 101 (1987) also defines a torque window--both minimum and maximum--that the torque for a specific fastener must meet. Such fasteners, used typically in the automotive industry and particularly on large over highway trucks, have been conventionally fabricated as all-metal, self-locking fasteners, where the locking element is in the form of distorted threads or a distorted metal body. These all-metal, self-locking fasteners have created numerous problems in the trucking industry. Typically, the distorted thread concept requires a cadmium plating but even here, when driven into an assembly, there is a metal to metal galling effect that takes place resulting in varying installation torque levels which, in turn, lead to over-tightening or under-tightening and, therefore, in lost production due to replacement of faulty joints. If the cadmium plating is replaced with other platings, typically zinc, the conditions are exacerbated. Much higher clamping torques are required to attain the same clamp loads.
All-metal locknuts also remove or damage the protective coating on the mating external threads; thus, resulting in bare metal exposure to the elements with the end result being oxidation or rusting effects. Additionally, over torquing sometimes causes the nut to be driven into the frame assembly resulting in scarring or scraping to the affected area, which requires removal of the assembly, reworking the damaged area and reinsertion back into production. This results in lost production, higher assembly costs and poor quality.
The plastic patch type, self-locking internally threaded fasteners of the present invention eliminate the above problems by meeting, or even exceeding, the IFI torque requirements of specifications 100/107 and 101 for all-metal cadmium plated nuts--even when the fasteners are zinc plated. Additionally, the self-locking fasteners of the present invention provide improved functional performance with a fastener that is lighter in weight and lower in cost than a corresponding all-metal locknut.